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SubjectRe: [RFC v4][PATCH 4/9] Memory management (dump)


Dave Hansen wrote:
> On Tue, 2008-09-09 at 03:42 -0400, Oren Laadan wrote:
>> + while (addr < end) {
>> + struct page *page;
>> +
>> + /*
>> + * simplified version of get_user_pages(): already have vma,
>> + * only need FOLL_TOUCH, and (for now) ignore fault stats.
>> + *
>> + * FIXME: consolidate with get_user_pages()
>> + */
>> +
>> + cond_resched();
>> + while (!(page = follow_page(vma, addr, FOLL_TOUCH))) {
>> + ret = handle_mm_fault(vma->vm_mm, vma, addr, 0);
>> + if (ret & VM_FAULT_ERROR) {
>> + if (ret & VM_FAULT_OOM)
>> + ret = -ENOMEM;
>> + else if (ret & VM_FAULT_SIGBUS)
>> + ret = -EFAULT;
>> + else
>> + BUG();
>> + break;
>> + }
>> + cond_resched();
>> + ret = 0;
>> + }
>
> get_user_pages() is really the wrong thing to use here. It makes pages
> *present* so that we can do things like hand them off to a driver. For
> checkpointing, we really don't care about that. It's a waste of time,
> for instance to perform faults to fill the mappings up with zero pages
> and page tables. Just think of what will happen the first time we touch
> a very large, very sparse anonymous area. We'll probably kill the
> system just allocating page tables. Take a look at the comment in
> follow_page(). This is a similar operation to core dumping, and we need
> to be careful.
>
> This might be fine for a proof of concept, but it needs to be thought
> out much more thoroughly before getting merged. I guess I'm
> volunteering to go do that.

The intention is not to allocate unallocated pages, but to get the page
pointer and bring in swapped out pages if necessary. (Avoiding swap-in
is possible, but left for future optimization).

Indeed, follow_page() does the work just fine; Of course, it should be
called with FOLL_ANON instead of FOLL_TOUCH. Thanks for pointing out.

Oren.


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